Method for preparing particles of alkali metal bicarbonate

ABSTRACT

A method for preparing particles of alkali metal bicarbonate by crystallization from a solution of alkali metal carbonate and/or bicarbonate in the presence of an additive in the solution, selected from the sulfates, sulfonates, the polysulfonates, the amines, the hydroxysultaines, the polycarboxylates, the polysaccharides, the polyethers and the ether-phenols, alkali metal hexametaphosphate, the phosphates, the sulfosuccinates, the amidosulfonates, the aminosulfonates, preferably selected from the polycarboxylates having a mean molecular weight lower than 8000 g/mol, and such that the additive is present in the solution at a concentration of at least 1 ppm and preferably of at most 200 ppm.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage entry under 35 U.S.C. § 371 of International Application No. PCT/EP2014/063559 filed Jun. 26, 2014, which claims priority to French Application No. 13.56160 filed Jun. 26, 2013, the whole content of this application being incorporated herein by reference for all purposes.

FIELD OF TECHNOLOGY

The invention relates to a method for preparing alkali bicarbonate particles and to the alkali bicarbonate particles obtainable by said method. The invention also relates to a method carried out for the purpose of controlling the bulk density of powders consisting of said particles, and/or carried out for the purpose of controlling the size of said particles.

PRIOR ART

The alkali metal bicarbonates, such as sodium bicarbonate (NaHCO₃) and potassium bicarbonate (KHCO₃), are products that have numerous properties, and are of interest in wide areas of use, extending from the pharmaceutical industry, to human and animal nutrition, passing through detergents, treatment of acid flue gases, and treatment of nonferrous metals.

The most common means for manufacturing these bicarbonates in the form of solid particles consists of crystallization of the bicarbonates by carbonation, with carbon dioxide, of a solution of carbonate of the corresponding alkali metal (sodium carbonate or potassium carbonate for example) or of a solution of hydroxide of the corresponding alkali metal (sodium hydroxide or potassium hydroxide for example). Generally these solutions are aqueous. However, water/alcohol solvent mixtures such as water/ethanol mixtures are also often used. The bicarbonates are often also crystallized by controlled cooling of solutions of bicarbonates, or by evaporation of such solutions.

Most of the uses of alkali bicarbonates in the form of particle powders require control of their bulk density (designated ‘BD’ in the present text). This control is important so as to be able to control the weight that can be put in given packaging (bags, big-bags, for example) or to control the weight loaded in bulk transport trucks that have fixed standard volumes. Moreover, the industrial users of such powders often have volumetric metering apparatus which has to be adjusted if the density of the powders delivered changes, and this causes loss of time if using different batches or batches from various production sites. It is therefore desirable to have a simple means in production of such bicarbonates for adjusting their BD.

Furthermore, the end users are interested in defined granulometric fractions that are optimized for their applications as a function of the required properties: for example dissolution rates, capacity for mixing with other powders homogeneously without segregation, flowability of the powders for ease of handling (measured for example by the angle of repose, or by the flow rate through a calibrated orifice), etc.

It is therefore important to have a simple and effective means so as to be able to adjust the granulometric fraction of the bicarbonate particles manufactured.

In the existing methods, several parameters can be adjusted in order to control the density of the powders obtained as well as their granulometries. However, these two characteristics are not in a one-to-one relationship. In fact, the bulk density depends on several factors, among which we may mention: the size of the particles, the granulometric distributions (distributions by weight of the particles in different granulometric fractions, unimodal or multimodal distributions, wide or narrow granulometric distributions), the form factors of the particles (roundness, robustness, size of greatest concavity, surface concavity index), etc.

Once again it is useful to have a simple means in production of such bicarbonates, for adjusting the size of the particles and notably their oversize or undersize on sieves with different cutoff diameters.

SUMMARY OF THE INVENTION

The present invention relates to a method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution designated hereinafter: ‘the solution’) in the presence of an additive in the solution, characterized in that:

the additive is selected from the following compounds:

-   -   sulfates, in particular sodium sulfate and organosulfates;     -   sulfonates;     -   polysulfonates;     -   amines, in particular primary amines, cyclic amines, quaternary         amines comprising at least one carboxylic acid group;     -   hydroxysultaines;     -   polycarboxylates;     -   polysaccharides;     -   polyethers and ether-phenols;     -   sulfosuccinates;     -   amidosulfonates;     -   aminosulfonates;     -   alkali metal hexametaphosphate in particular of sodium or         potassium;     -   phosphates, in particular organophosphates, and phosphonates;

and the additive is present in the solution at a concentration of at least 1 ppm, advantageously at least 5 ppm, more advantageously at least 10 ppm.

The inventors in fact found, surprisingly, that the additives listed above, used during crystallization of the alkali metal bicarbonate particles, made it possible to obtain particles having a synergy between their granulometry, their form factor, or even their surface electric charge potentials, allowing either a large, adjustable increase in bulk density of the particles obtained, or a decrease in density. They also make it possible to modulate their granulometries, and their widening or narrowing of the granulometric distributions.

The invention also relates to the alkali metal bicarbonate particles thus obtained, and/or obtainable by the present method.

DEFINITIONS

In the present specification, selection of an element from a group of elements also describes explicitly:

selection of two or selection of several elements of the group,

selection of an element from a subgroup of elements that consists of the group of elements from which one or more elements have been removed.

Moreover, it has to be understood that the elements and/or the characteristics of an apparatus, a method or a use, described in the present specification may be combined in all possible ways with the other elements and/or characteristics of the apparatus, method, or use, explicitly or implicitly, while remaining within the scope of the present specification, and must be considered as forming an integral part of the present specification.

In the passages of the present specification given below, different embodiments, or variants of the invention are defined in more detail. Each embodiment or variant thus defined may be combined with another embodiment, or with another variant, and this for each embodiment or variant unless stated otherwise or manifestly incompatible when the range of one and the same value parameter is unconnected. In particular, any variant indicated as being preferred or advantageous may be combined with another variant or with the other variants indicated as being preferred or advantageous.

In the present specification, the description of a range of values for a variable, defined by a lower limit, or an upper limit, or by a lower limit and an upper limit, also comprises the embodiments where the variable is selected respectively from the range of values: excluding the lower limit, or excluding the upper limit, or excluding the lower limit and the upper limit.

In the present specification, the description of several successive ranges of values for one and the same variable also comprises the description of embodiments where the variable is selected from any other intermediate range included in the successive ranges. Thus, for example when it is described that the quantity X is generally above 10, advantageously above 15″, the present description also describes the embodiment where: the quantity X is above 11″, or also the embodiment where: the quantity X is above 13.74″, etc.; 11 or 13.74 being values between 10 and 15.

The term “comprising” includes “consisting essentially of” as well as “consisting of”.

In the present specification, the use of “a” or one in the singular, also comprises the plural (“some”), and vice versa, unless the context clearly indicates the opposite. As an example, “an additive” denotes one additive or more than one additive.

If the term “about” is used before a quantitative value, the latter corresponds to a variation of ±10% of the nominal quantitative value, unless stated otherwise.

The term “average” denotes a number average, unless stated otherwise.

The term “ppm” denotes parts per million expressed by weight (for example 1 ppm=1 mg per kilogram).

The term ‘granulometric spread’, or ‘span’, of a granulometric distribution of particles corresponds in the present specification to the following ratio: (D90−D10)/D50, the term D90 denoting the diameter for which 90 wt % of the particles have a size less than or equal to D90, the term D50 denoting the diameter for which 50 wt % of the particles have a size less than or equal to D50 (average size by weight), the term D10 denoting the diameter for which 10 wt % of the particles have a size less than or equal to D10.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution called hereinafter: ‘the solution’) in the presence of an additive in the solution, for modifying:

the granulometric distribution of the particles, in particular the fraction of ‘fine’ particles under 125 μm, or the fraction of ‘coarse’ particles above 250 μm, or the ‘granulometric spread’ (or ‘span’)

or their BD,

or their dissolution times,

or their angles of repose,

or their flow rate,

or their abrasion index.

In fact the inventors discovered that the various additives mentioned hereunder are modifiers of crystallization of the crystals of alkali metal bicarbonates that may be used in a small amount in a solution of alkali metal carbonate and/or bicarbonate, in which the crystals of alkali bicarbonate are crystallized.

In the present specification, ‘alkali metal bicarbonate particles’ means particles comprising at least 60%, advantageously at least 75%, more advantageously at least 85%, even more advantageously at least 90% or at least 95% or at least 99% of alkali metal bicarbonate. Generally in the present invention, the alkali metal bicarbonate particles comprise at most 40%, advantageously at most 25%, more advantageously at most 15%, even more advantageously at most 10% or at most 5% or at most 1% of alkali metal carbonate. In the present invention, the alkali metal particles may also contain small contents of ammonium bicarbonate, in general at most 2%, or at most 1% or at most 0.7% of ammonium bicarbonate. This is the case notably when the particles are crystallized in solutions containing ammonia, such as in the crystallization of sodium bicarbonate in the Solvay ammonia process, for manufacture of sodium carbonate. The alkali metal bicarbonate particles obtained in this method are generally denoted by the expression ‘crude bicarbonate’ (cf. Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH Verlag GmbH & Co, Weinheim, “Sodium carbonate”, Vol. 33 page 307).

In the present invention, the alkali metal bicarbonate may be bicarbonate of: lithium, sodium, potassium, rubidium or cesium. Advantageously the alkali metal bicarbonate is sodium bicarbonate or sodium bicarbonate. More advantageously the alkali metal bicarbonate is sodium bicarbonate. In particular, in the present invention, the alkali metal bicarbonate particles are advantageously particles comprising at least 99% of sodium bicarbonate and less than 1% of sodium carbonate or water.

The present invention is based on the discovery of the effects of the additives mentioned in the present specification, for which the different variants of the method and/or of the product obtained by said variants of the method are described in more detail below.

Item 1. Method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution designated hereunder: ‘the solution’) in the presence of an additive in the solution, characterized in that:

the additive is selected from the following compounds:

-   -   sulfates, in particular sodium sulfate and organosulfates;     -   sulfonates;     -   polysulfonates;     -   amines, in particular primary amines, cyclic amines, quaternary         amines comprising at least one carboxylic acid group;     -   hydroxysultaines;     -   polycarboxylates;     -   polysaccharides;     -   polyethers and ether-phenols;     -   sulfosuccinates;     -   amidosulfonates;     -   aminosulfonates;     -   alkali metal hexametaphosphate in particular of sodium or of         potassium;     -   phosphates, in particular organophosphates, and phosphonates;

and the additive is present in the solution at a concentration of at least 1 ppm, advantageously of at least 5 ppm, more advantageously of at least 10 ppm.

Item 2. Method according to item 1, in which the additive is an organosulfate and is selected from the following compounds:

-   -   alkyl sulfates comprising from 4 to 20 carbons, preferably from         8 to 16 carbons, or from 8 to 14 carbons such as lauryl sulfate         (dodecyl sulfate),     -   alkyl ether sulfates (such as Rhodapex ESB-70/A2);

Item 3. Method according to item 1, in which the additive is a sulfonate and is selected from the following compounds:

-   -   alkyl sulfonates;     -   alkylbenzene sulfonates or alkylnaphthalene sulfonates; in         particular disodium dodecyl(sulfonatophenoxy) benzene-sulfonate         (CAS 28519-02-0, such as Rhodacal DSB), disodium 2,2′(or         3,3′)-oxybis[5(or 2)-dodecylbenzenesulfonate (CAS 25167-32-2),         sodium alkyl naphthalene sulfonate (such as Rhodacal BX-78),         isopropylamine dodecylbenzene sulfonate (such as Rhodacal 330);     -   polysulfonates, in particular polyvinylsulfonic acid and alkali         metal salts thereof;     -   aminosulfonates in particular: N-methyltaurine, sodium         N-methyltaurinate, isopropylamine dodecylbenzene sulfonate;     -   amidosulfonates in particular those with more than 6 carbons, in         particular: sodium 2-[methyloleoylamino]ethane-1-sulfonate (such         as Geropon T/77);     -   sulfosuccinates in particular sodium docusate (such as Geropon         SDS, Aerosol OT, Constonate, Diomedicone, Clestol, Complemix,         Defilin, Dioctlyn, Dioctylal, Diosuccin);

Item 4. Method according to item 1, in which the additive is an amine, and is selected from the following compounds:

-   -   primary amines, in particular hexamethylenediamine;     -   quaternary amines comprising at least one carboxylic acid group,         in particular the quaternary amines of formula: (H₃C)₃—N—R—COOH         with R=(CH₂)_(n), and n=1 to 4, in particular trimethylglycine         (or betaine);     -   quaternary amines comprising at least one sulfonate group, in         particular hydroxysultaines, such as cocamidopropyl         hydroxysultaine (such as Mirataine CBS);     -   cyclic amines (such as Miranol DM);

Item 5. Method according to item 1, in which the additive is a polycarboxylate, and is preferably selected from the following compounds: polyacrylates, in particular crosslinked polyacrylates, or copolymers of acrylic and maleic acids, or of sodium prop-2-enoate (such as: Polyco, Rhotex GS, Acrysol lmw-45N, Hiviswako 105); in particular polyacrylates or acrylic and maleic copolymers, or alkali metal salts thereof, of average molecular weight below 8000 g/mol, or greater than or equal to 8000 g/mol;

Item 6. Method according to item 1, in which the additive is a polysaccharide, and is preferably selected from the following compounds:

-   -   guar gums and their derivatives, in particular hydroxypropylguar         (such as Jaguar HP-105);     -   alginic acid and its salts such as of sodium or of calcium or of         copper (such as: Kaltostat, Calginat, Landalgine, Kalrostat,         Kelacid, Vocoloid, Xantalgin);     -   carboxymethylcellulose (such as Aquaplast, Carmethose, Cellofas,         Cellpro, Cellugel, Collowel, Ethoxose, Orabase, Lovosa);

Item 7. Method according to item 1, in which the additive is a polyether or an ether-phenol or a polyaromatic ester, and is preferably selected from the following compounds:

-   -   polyethylene glycol mono(tristyrylphenyl) ether;     -   ethoxylated oleyl alcohols (such as Rhodasurf ON/870-E);     -   branched octylphenoxy poly(ethyleneoxy)ethanols (such as Igepal         CA630);     -   ethoxylated polyarylphenols, in particular ethoxylated         tristyrylphenol (such as Soprophor CY/8);

Item 8. Method according to item 1, in which the additive is a sulfosuccinate, and is preferably selected from the following compounds:

-   -   sulfosuccinates, in particular: sodium docusate (such as Aerosol         OT, Constonate, Diomedicone, Clestol, Complemix, Defilin,         Dioctlyn, Dioctylal, Diosuccin);

Item 9. Method according to item 1, in which the additive is an organophosphate or a phosphonate, and is preferably selected from the following compounds:

-   -   polyoxyethylene alkylether phosphates, whose alkyl chain         advantageously comprises from 6 to 20 carbons, in particular         polyoxyethylene octadecenyl ether phosphate (such as Lubrophos         LB/400-E);     -   poly(oxy-1,2-ethanediyl),alpha.-(dinonylphenyl)-omega-hydroxy-phosphate         (such as RHODAFAC RM-510);     -   alkyl-phosphonic or hydroxyalkyl-phosphonic acids of         semi-structural formula ((HO)₂OP)—R with R an alkyl group, or         hydroxyalkyl group, in particular with R an alkyl group with 1         to 3 carbons, or R a hydroxyalkyl group comprising 1 to 3         carbons and 1 to 2 hydroxyl (—OH) groups, or salts thereof in         particular of alkali metal such as sodium or potassium;     -   alkyl-diphosphonic acids, in particular those of semi-structural         formula ((HO)₂OP)—R—(PO(OH)₂) and with R an alkyl group, or a         hydroxyalkyl group, in particular with R an alkyl group with 1         to 3 carbons, or R a hydroxyalkyl group comprising 1 to 3         carbons and 1 to 2 hydroxyl (—OH) groups (such as etidronic acid         ((HO)₂OP)—C(CH₃)OH—(PO(OH)₂)), or salts thereof in particular of         alkali metal such as sodium or potassium;     -   nitrilo-alkyl-phosphonic acids of semi-structural formula         ((HO)₂OP—R)₃—N with R an alkyl chain with 1 to 5 carbons, or         salts thereof in particular of alkali metal such as sodium or         potassium, in particular nitrilo-tri-(methanephosphonic) acid or         salts thereof in particular of alkali metal such as sodium or         potassium.

Item 10. Method according to any one of items 1 to 9, in which the additive is present at a concentration of at most 200 ppm, advantageously of at most 100 ppm, more advantageously of at most 70 ppm.

Item 11. Method according to any one of items 1 to 10, in which crystallization of alkali metal bicarbonate particles is carried out by cooling the solution.

Item 12. Method according to item 11, in which cooling of the solution is carried out between 70° and 30° C.

Item 13. Method according to any one of items 1 to 12, in which crystallization of alkali metal bicarbonate particles is carried out by carbonation of the solution with carbon dioxide.

Item 14. Method according to item 13, in which carbonation of the solution is carried out at a temperature of at least 20° C., advantageously at least 30° C., and preferably of at most 95° C., more preferably of at most 90° C.

Item 15. Method according to any one of items 1 to 14, in which crystallization of alkali metal bicarbonate particles is carried out by evaporation of at least a proportion of the solution.

Item 16. Method according to any one of items 1 to 15, in which crystallization of alkali metal bicarbonate particles is carried out by concomitant cooling and carbonation of the solution; or by concomitant cooling and carbonation and evaporation of the solution.

Item 17. Method according to any one of items 13, or 16, in which carbonation of the solution with carbon dioxide is carried out with a gas comprising: 10 to 100% of CO₂ by volume based on dry gas, advantageously at least 20% of CO₂ by volume based on dry gas, more advantageously at least 30% of CO₂ by volume based on dry gas, even more advantageously at least 35% of CO₂ by volume based on dry gas.

Item 18. Method according to any one of items 1 to 17 carried out at a temperature between 20° C. and 95° C., preferably between 30 and 90° C.

Item 19. Method according to any one of items 1 to 18, in which addition of the additive is carried out for controlling or modifying:

-   -   the granulometry (such as D10, D50, D90 or the span), or     -   the undersize or oversize on a sieve at 125 μm, or     -   the undersize or oversize on a sieve at 250 μm.         of the alkali metal bicarbonate particles obtained after         separation of the crystallization solution and of the alkali         bicarbonate particles.

Item 20. Method according to any one of items 1 to 19, in which addition of the additive is carried out for controlling or modifying:

-   -   the dissolution time, or     -   the bulk density (BD), or     -   the flow rate in a calibrated orifice,     -   the abrasion index,         of the alkali metal bicarbonate particles obtained after         separation of the crystallization solution and of the alkali         bicarbonate particles, then drying of the alkali metal         bicarbonate particles.

Item 21. Method according to item 20 according to which addition of the additive makes it possible to increase the BD, and the increase in the BD of the alkali metal bicarbonate particles is at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 100% relative to the BD of alkali metal bicarbonate particles obtained in the same conditions of crystallization but without addition of additive.

Item 22. Method according to item 21 according to which addition of the additive makes it possible to reduce the BD, and the reduction of the BD of the alkali metal bicarbonate particles is at least 10%, or at least 15% relative to the BD of alkali metal bicarbonate particles obtained in the same conditions of crystallization but without addition of additive.

Item 23. Method according to item 19 carried out for controlling the size of the alkali bicarbonate particles, in particular for controlling the sieve undersize at 125 μm or sieve oversize at 250 μm of the alkali bicarbonate particles.

Item 24. Method according to any one of items 19 to 21, in which addition of additive makes it possible to:

-   -   increase the granulometry (such as D10, D50, D90) or reduce the         span, or     -   reduce the sieve undersize at 125 μm, or     -   increase the sieve oversize at 250 μm, or     -   reduce the dissolution time, or     -   increase the bulk density (BD), or     -   reduce the abrasion index,         of the alkali metal bicarbonate particles relative to one and         the same preparation that would be performed without additive,         and in said method the additive is selected from the additives         listed in item 6 (the polysaccharides), in particular alginic         acid or carboxymethylcellulose.

Item 25. Method according to item 24, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation.

Item 26. Method according to any one of items 19 to 21, in which addition of additive makes it possible to:

-   -   increase the granulometry (such as D10, D50, D90) or reduce the         span, or     -   reduce the sieve undersize at 125 μm, or     -   increase the sieve oversize at 250 μm, or     -   reduce the dissolution time, or     -   increase the bulk density (BD), or     -   reduce the abrasion index,         of the alkali metal bicarbonate particles relative to one and         the same preparation effected without additive, and in which         said method the additive is selected from the additives listed         in item 5 (polycarboxylates) and selected from: polyacrylates,         in particular crosslinked, or acrylic and maleic acid copolymers         or alkali metal salts thereof, of average molecular weight         greater than or equal to 8000 g/mol.

Item 27. Method according to any one of items 19, 20 or 22, in which addition of additive makes it possible to:

-   -   reduce the granulometry (such as D10, D50, D90) or increase the         span, or     -   increase the sieve undersize at 125 μm, or     -   reduce the sieve oversize at 250 μm, or     -   reduce the dissolution time, or     -   reduce the bulk density (BD), or     -   increase the abrasion index,         of the alkali metal bicarbonate particles relative to one and         the same preparation effected without additive, and in which         said method the additive is selected from the additives listed         in item 5 (polycarboxylates) and selected from: polyacrylates,         in particular crosslinked, or acrylic and maleic acid copolymers         or alkali metal salts thereof, of average molecular weight below         8000 g/mol.

Item 28. Method according to item 26 or 27, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation.

Item 29. Method according to any one of items 19, 20 or 22, in which addition of additive makes it possible to:

-   -   reduce the granulometry (such as D10, D50, D90) or increase the         span, or     -   increase the sieve undersize at 125 μm, or     -   reduce the sieve oversize at 250 μm, or     -   reduce the dissolution time, or     -   reduce the bulk density (BD), or     -   increase the abrasion index,         of the alkali metal bicarbonate particles relative to one and         the same preparation effected without additive, and in which         said method the additive is selected from the additives listed         in item 9 (an organophosphate or a phosphonate).

Item 30. Method according to item 29, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation.

Item 31. Alkali metal bicarbonate particles obtainable by the method according to any one of items 1 to 30 and comprising at least 10 ppm, advantageously at least 20 ppm of the additive.

The additives are used in the present invention, in general, at a concentration in the solution of at least 1 ppm, advantageously at least 5 ppm, more advantageously at least 10 ppm.

The amounts in ppm (parts per million) are expressed by weight and correspond to the amount of additive introduced into the solution if crystallization is effected in a batch (discontinuous) process or corresponds to the proportion of additive introduced relative to the solution fed into the crystallization apparatus if crystallization is carried out continuously. They correspond to the mg of active substance of additive per kg of solution.

A low concentration of these additives is sufficient to have a remarkable effect on the bicarbonate particles obtained. However, a concentration of additives that is too high adversely affects the purity of the alkali metal bicarbonates. In the present invention the additive is present at a concentration of at most 200 ppm, advantageously of at most 100 ppm, more advantageously of at most 70 ppm, even more advantageously of at most 50 ppm.

Amounts of about (i.e. ±10%) of: 10 ppm, or 20 ppm, or 50 ppm of additive in the solution are particularly advantageous.

However, the amounts of additives may be adjusted in order to adjust the desired value of the parameter, for example BD, or the parameter of the granulometric sieve oversize, in particular at 125 μm or at 250 μm.

The solution of alkali metal carbonate and/or bicarbonate is preferably an aqueous solution comprising from 1 to 30 wt % of alkali metal carbonate and/or comprising 1 to 18 wt % of alkali metal bicarbonate.

Crystallization of the particles according to the invention may be done with seeding or without seeding of crystals of alkali metal bicarbonate.

In a preferred embodiment according to the invention, the solution is seeded. In the case of batch crystallization with seeding, an amount of seeding of the order of 0.1 to 10% of bicarbonate crystals is introduced before the start of crystallization of the bicarbonate particles according to the invention. In the case of crystallization carried out continuously, the suspension density of the bicarbonate crystals executing production in the crystallization apparatus constitutes the seeding itself. In this case, suspension densities of alkali metal bicarbonate particles relative to the weight of the suspension of particles in the crystallization mother liquor are generally from 0.1 to 25 wt %.

In a first embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by cooling the solution.

In a second embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by carbonation of the solution with carbon dioxide.

In a third embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by evaporation of at least a portion of the solution.

Each of the three embodiments mentioned above may be combined in the present invention. In particular, in a fourth embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by concomitant cooling and carbonation of the solution; or by concomitant cooling and carbonation and evaporation of the solution. In the case when crystallization is effected by carbonation with a gas comprising carbon dioxide, injection of the gas might not be saturated by the solvent of the solution at the temperature of the suspension of the crystallizing particles. In this case, injection of the gas is accompanied by partial evaporation of the solvent of the solution. This also causes cooling of the solution.

In each of the above embodiments, it is preferred in the present invention when at least two liquid and gaseous phases are present during preparation of particles by crystallization, when the continuous phase is the liquid phase and when the gas phase is a dispersed phase in the liquid phase.

In each of the above embodiments when there is carbonation of the solution, carbonation of the solution with carbon dioxide is carried out with a gas generally comprising: 10 to 100% of CO₂ by volume based on dry gas, advantageously at least 20% of CO₂ by volume based on dry gas, more advantageously at least 30% of CO₂ by volume based on dry gas, even more advantageously at least 35% of CO₂ by volume based on dry gas

In the present invention, preparation of alkali metal bicarbonate particles by crystallization is generally carried out at a temperature between 20° C. and 95° C. It is advantageous for the temperature to be at least 30° C. It is also advantageous for the temperature to be at most 90° C.

The bulk density of the alkali metal bicarbonate particles is measured by the following procedure: the particles are put in a smooth steel hopper of truncated shape (top diameter 53 mm, bottom diameter 21 mm, height 58 mm) closed at the bottom by a closing device consisting of a horizontal sheet of smooth steel. Then the bottom of the hopper is opened by removing the closing device, and the particles are collected in a 50 cm³ cylinder (inside diameter 37 mm, height 46 mm) situated (its top) less than 5 cm under the closing device. Then the excess of particles projecting above the cylinder is leveled, without tamping the powder, and checking that the whole cylinder is filled with powder to the brim, the weight of the powder contained in the 50 cm³ is weighed and the (“free”) bulk density of the powder is expressed as the apparent weight of the powder referred to the volume of 50 cm³, expressed in kg/liter or kg/m³.

The following examples are given to illustrate certain embodiments of the present invention. They are not limiting in any case.

EXAMPLES Example 1—Crystallization of Sodium Bicarbonate Particles by Cooling—Control of the Free Bulk Density

Equipment used: 500-ml double-jacket beakers, stirred by a magnetic bar at 350 rev/min. Buchner filter and laboratory vacuum pump.

Operating Conditions:

TABLE 1 Operating conditions of the tests. Initial aqueous solution Concentrations [NaHCO₃] [Na₂CO₃] Seeding Stirring required required Type Amount speed g/kg g/kg — g rev/min 117 70 BICAR 1 350 200-250 μm

The “BICAR” used as seed is a sodium bicarbonate of brand “BICAR® Codex” of Pharmacopeia quality, from the SOLVAY company sieved between 200 and 250 μm.

TABLE 2 Operating conditions of the tests (continued). Temperature Rate of Maturation Initial decrease Final time at 30° C. ° C. ° C./h ° C. min 70 40 30 30

At the end of the step of crystallization by cooling, the suspension of bicarbonate particles is filtered on a Buchner and washed with 50 ml of ethanol saturated with sodium bicarbonate, to remove most of the crystallization mother liquor. The particles are then dried in open air overnight. The agglomerates due to drying are broken up on a stainless steel plate with a rubber stopper with light hand pressure, and the powder obtained is then sieved at 500 μm.

Results of the Reference Tests without Additive

TABLE 3 BD of the particle powders without additive. BD Test Additive g/L 12-2 No additive 403 12-3 No additive 422 12-4 No additive 425 12-5 No additive 423 12-6 No additive 452 22-1 No additive 436 22-2 No additive 426 22-3 No additive 439 22-4 No additive 412 22-5 No additive 420 22-6 No additive 443 MEAN BD 427 STANDARD 14 DEVIATION CV 3%

The following table (Table 4.a) only gives the additives for which the BD is greater than the mean BD of the reference tests+3 standard deviations (i.e. >470 kg/m³). Moreover, the ratio BD add/BD ref is the ratio of the BD of the sodium bicarbonate crystallized in the presence of additive divided by the mean BD of the sodium bicarbonate crystallized without additive (i.e. 427 kg/m³):

TABLE 4.a BD of the particle powders with additive and comparison with and without additive. BD add/BD [Additive] BD ref Test Additive mg/kg g/L — 13-3 Betaine 20 483 1.1 24-6 Mirataine CBS 20 486 1.1 31-5 Rhodacal DSB 20 486 1.1 31-2 Rhodacal DSB 20 488 1.1 15-4 Polyvinysulfonic acid 20 489 1.1 19-5 Sodium polyacrylate 20 492 1.2 16-1 Alginic acid 20 498 1.2 13-4 Carboxymethylcellulose 20 501 1.2 15-6 Polyvinysulfonic acid 20 502 1.2 19-2 Sodium polyacrylate 20 504 1.2 27-2 Soprophor CY/8 20 506 1.2 16-3 Alginic acid 20 513 1.2 19-1 Soybean lecithin 20 525 1.2 37-5 Rhodacal BX-78 20 529 1.2 37-2 Rhodacal BX-78 20 532 1.2 40-2 Sodium 20 539 1.3 hexametaphosphate 26-5 Geropon SDS 20 542 1.3 37-4 Rhodacal 330 20 552 1.3 19-3 Soybean lecithin 20 552 1.3 26-2 Geropon SDS 20 559 1.3 35-2 Rhodacal DSB 50 568 1.3 40-4 Rhodacal BX-78 50 571 1.3 42-3 Sodium 20 574 1.3 hexametaphosphate 37-6 Rhodacal 330 20 575 1.3 40-6 Rhodacal BX-78 50 576 1.3 42-6 Sodium 20 577 1.4 hexametaphosphate 42-5 Sodium 20 578 1.4 hexametaphosphate 19-6 Sodium 20 587 1.4 hexametaphosphate 40-1 Sodium 20 592 1.4 hexametaphosphate 35-5 Rhodacal DSB 50 596 1.4 42-1 Sodium 20 598 1.4 hexametaphosphate 19-4 Sodium 20 600 1.4 hexametaphosphate 40-3 Sodium 20 603 1.4 hexametaphosphate 26-4 Geropon T/77 20 609 1.4 42-4 Sodium 20 624 1.5 hexametaphosphate 18-2 Polyvinysulfonic acid 50 640 1.5 41-4 Rhodacal 330 50 640 1.5 26-6 Geropon T/77 20 644 1.5 18-5 Polyvinysulfonic acid 50 646 1.5 41-6 Rhodacal 330 50 664 1.6 21-4 Polyvinysulfonic acid 50 666 1.6 18-1 Sodium dodecyl 10 686 1.6 sulfate 21-6 Polyvinysulfonic acid 50 692 1.6 18-4 Sodium dodecyl 50 722 1.7 sulfate 14-4 Sodium dodecyl 20 759 1.8 sulfate 18-3 Sodium dodecyl 10 765 1.8 sulfate 18-6 Sodium dodecyl 50 810 1.9 sulfate 14-6 Sodium dodecyl 20 839 2.0 sulfate

TABLE 4.b BD (kg/m³) of the particle powders obtained with additives of the polyacrylate type of different molecular weights (average molecular weights). BD (kg/m³) - Function of the molecular weight of the sodium polyacrylate and the amount of additive added Molecular weight of Na polyacrylate (g/mol) 10 ppm 20 ppm 30 ppm 50 ppm 1800 322 296 300 273 2100 367 345 — 268 8000 464 504 560 630 14 800   458 582 644 898 1 250 000    442 477 553 697 Remarks:

-   -   For 10 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   600<BD<700 kg/m³ (factor 1.4 to 2.0) using sodium dodecyl             sulfate     -   For 20 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   470<BD<600 kg/m³:         -   Betaine         -   Mirataine CBS         -   Rhodacal DSB         -   Polyvinysulfonic acid         -   Sodium polyacrylate of average molecular weight≧8000 g/mol         -   Alginic acid         -   Carboxymethylcellulose         -   Soprophor CY/8         -   Rhodacal BX-78         -   Sodium hexametaphosphate         -   Geropon SDS         -   Rhodacal 330         -   Soybean lecithin         -   Geropon T/77         -   600<BD<700 kg/m³:         -   Geropon T/77         -   Sodium hexametaphosphate.         -   BD>700 kg/m³:         -   Sodium dodecyl sulfate.     -   For 50 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   470<BD<600 kg/m³ using Rhodacal DSB, Rhodacal BX-78         -   630<BD<898 kg/m³ using sodium polyacrylate of average             molecular weight at least 8000 g/mol         -   600<BD<700 kg/m³ using polyvinysulfonic acid, Rhodacal 330         -   BD>700 kg/m³ using sodium dodecyl sulfate.

The following table (Table 4.c) only gives the additives for which BD is below the mean BD of the reference tests minus 1.4 standard deviations (i.e. 407 g/L).

TABLE 4.c BD of the particle powders with additive (organophosphates and phosphonates) and comparison with and without additive. BD add/BD [Additive] BD ref Test Additive mg/kg g/L — Lubrophos LB/400-E 20 404 0.95 Rhodafac RM 510 20 407 0.95 Nitrile-tri-(methane- 20 380 0.89 phosphonic) acid Etidronic acid 20 369 0.86

Comments for the organophosphates and phosphonates:

-   -   For 20 ppm of additive (ppm=mg active substance/kg initial         liquid):         369<BD<407 kg/m³ (factor 0.86 to 0.95) using organophosphates or         phosphonates; i.e. decreases by 5 to 14% of BD relative to the         reference BD.

Example 2—Crystallization of Sodium Bicarbonate Particles by Cooling—Control of the Sieve Oversize

Equipment and operating conditions: same as in example 1 (cf. notably Tables 1 and 2).

Results of the Reference Tests without Additive:

TABLE 5 Results of the reference tests without additive - Oversize on vibratory sieve for particle powders. Fraction <125 μm 125 < F < 250 μm F >250 μm Test Additive (wt %) (wt %) (wt %) 12-1 No additive 21 64 14 12-2 No additive 22 62 16 12-3 No additive 28 61 11 12-4 No additive 23 53 24 12-5 No additive 27 62 11 12-6 No additive 31 58 11 22-1 No additive 26 57 17 22-2 No additive 25 59 16 22-3 No additive 32 55 12 22-4 No additive 18 60 22 22-5 No additive 29 59 13 MEAN 26 59 15 STANDARD 4 3 5 DEVIATION Coef. of 17% 5% 31% Variation Results of the Tests with Additives:

The following table (Table 6) only gives the additives for which the undersize at 125 μm is greater than 38 wt % (>mean value of the reference tests without additive+3 standard deviations). Moreover, the ratio F add/F ref is the ratio of the undersize fraction by weight at 125 μm of sodium bicarbonate crystallized in the presence of additive divided by the average undersize fraction by weight at 125 μm of sodium bicarbonate crystallized without additive (i.e. 26 wt %):

TABLE 6 Results of the tests with additives - undersize on vibratory sieve for particle powders. F add/F [Additive] F <125 μm ref Test Additive mg/kg wt % — 30-5 Lubrophos LB/400-E 20 65 2.5 41-6 Rhodacal 330 50 62 2.4 28-6 Rhodafac RM 510 20 53 2.1 35-3 Lubrophos LB/400-E 50 53 2.0 14-6 Sodium dodecyl 20 53 2.0 sulfate 27-6 Repel-O-TEX SF2 20 53 2.0 18-3 Sodium dodecyl 10 50 1.9 sulfate 14-4 Sodium dodecyl 20 48 1.9 sulfate 21-5 Magnesium 20 47 1.8 41-4 Rhodacal 330 50 46 1.8 18-4 Sodium dodecyl 50 45 1.7 sulfate 28-3 Rhodasurf ON/870-E 20 44 1.7 32-3 Igepal CA 630 20 43 1.7 18-6 Sodium dodecyl 50 43 1.7 sulfate 34-3 Antarox L-62 20 42 1.6 27-3 Jaguar HP 105 20 42 1.6 37-6 Rhodacal 330 20 42 1.6

The following table (Table 7) only gives the additives for which the oversize at 250 μm is greater than the average oversize at 250 μm+3 standard deviations (30 wt %) of the sodium bicarbonates of the reference tests without additive. Moreover, the ratio F add/F ref is the ratio of the oversize fraction by weight at 250 μm of sodium bicarbonate crystallized in the presence of additive divided by the average oversize fraction by weight at 250 μm of sodium bicarbonate crystallized without additive (i.e. 15 wt %):

TABLE 7.a Results of the tests with additives - Oversize on vibratory sieve for particle powders. F add/F [Additive] F >250 μm ref Test Additive mg/kg wt % — 19-4 HMP 20 68 4.5 42-4 HMP 20 63 4.2 40-1 HMP 20 62 4.1 42-1 HMP 20 58 3.9 19-2 Sodium 20 58 3.8 polyacrylate 19-5 Sodium 20 56 3.7 polyacrylate 42-6 HMP 20 54 3.6 42-5 HMP 20 53 3.6 15-5 Hexamethylene- 20 53 3.5 diamine 40-2 HMP 20 53 3.5 42-3 HMP 20 52 3.5 19-6 HMP 20 51 3.4 15- Polyvinysulfonic 20 51 3.4 avr acid 40-3 HMP 20 47 3.2

The following table (Table 7.b) gives the specific results obtained with sodium polyacrylates of different average molecular weights between 1800 and 1 250 000 g/mol and with various amounts added: 10, 20, 30, 50 ppm.

TABLE 7.b Results of the tests with additives of the sodium polyacrylate type with different average molecular weights - Oversize on vibratory sieve for particle powders (F >250 μm in wt %). F >250 μm (wt %) - Function of the molecular weight of sodium polyacrylate and of the amount of additive added Average molecular weight Na Polyacrylate (g/mol) 10 ppm 20 ppm 30 ppm 50 ppm 1800 7 3 7 9 2100 4 2 3 8000 9 43 60 75 14 800   27 52 64 71 1 250 000    33 33 58 72

The following table (Table 8) only gives the additives for which the fraction of particles between 125 and 250 μm is greater than the mean fraction 125-250 μm+3 standard deviations of the reference tests without additive (i.e. 68 wt %). Moreover, the ratio F add/F ref is the ratio of the 125-250 μm fraction by weight of sodium bicarbonate crystallized in the presence of additive divided by the average 125-250 μm fraction by weight of sodium bicarbonate crystallized without additive (i.e. 59 wt %):

TABLE 8 Results of the tests with additives - Oversize on vibratory sieve for particle powders. F add/F [Additive] 125 < F < 250 μm ref Test Additive mg/kg wt % — 18-5 Polyvinysulfonic 50 76 1.3 acid 21-4 Polyvinysulfonic 50 72 1.2 acid 18-2 Polyvinysulfonic 50 70 1.2 acid Remarks:

-   -   For 10 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   Production of fines with the following additives:             -   Sodium dodecyl sulfate     -   For 20 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   Production of fines (F<125 μm) with the following additives:             -   Lubrophos LB/400-E             -   Rhodacal 330             -   Rhodafac RM 510             -   Sodium dodecyl sulfate             -   Repel-O-TEX SF2             -   Magnesium             -   Rhodasurf ON/870-E             -   Igepal CA 630             -   Antarox L-62             -   Jaguar HP105         -   Production of coarse particles (F>250 μm) with the following             additives:             -   Sodium hexametaphosphate             -   Sodium polyacrylate             -   Hexamethylenediamine             -   Polyvinysulfonic acid         -   For sodium polyacrylate, it can be seen from Table 7.b that             for the production of coarse particles there is a limiting             average molecular weight for the homopolymers of polyacrylic             acid. It is found in fact that starting from 8000 g/mol, the             granulometric fraction above 250 μm (F>250 μm) is almost             constant at the concentrations of additive tested.     -   For 50 ppm of additive (ppm=mg active substance/kg initial         liquid):         -   Production of fines (F<125 μm) with the following additives:             -   Rhodacal 330             -   Lubrophos LB/400-E             -   Sodium dodecyl sulfate         -   Production of a Bicar with reduced granulometric spread:             -   Polyvinysulfonic acid     -   For 20 ppm of additive (ppm=mg active substance/kg initial         liquid): it can be seen that the organophosphates such as:         -   Lubrophos LB/400-E         -   Rhodafac RM 510             significantly increase the number of fines passing through             the 125 μm sieve.

Example 3—Crystallization of Sodium Bicarbonate Particles by Carbonation

The present example illustrates continuous carbonation for manufacture of sodium bicarbonate with and without additives.

Operating Conditions:

The preparation of sodium bicarbonate particles by crystallization by carbonation is carried out in a closed precipitation reactor with useful capacity of 5 liters, stirred at 800 (referenced ‘a’) or 1070 revolutions per minute (referenced ‘b’). A feed solution containing 100 grams of sodium bicarbonate per kilogram of aqueous solution and 130 grams of sodium carbonate per kilogram of aqueous solution is injected continuously at the center of the stirred reactor at a flow rate of 3.3 liters per hour. Carbon dioxide is injected into the reactor at a flow rate of 750 normal liters per hour via a perforated pipe placed at the bottom of the reactor under the stirrer. The suspension formed from sodium bicarbonate particles is withdrawn continuously at the bottom of the reactor at a flow rate of 3.3 liters per hour in order to maintain a constant level in the reactor. The reactor for crystallization by carbonation is placed under relative pressure of the gas cover 200 to 600 mbar relative. The additive is prepared as a solution of 1 wt % of active substance in water in such a way that the amounts of additive are expressed here as weight of active substance per weight of solution to be crystallized. The solution of additive is injected into the center of the stirred suspension.

For the sodium polyacrylate, the average molecular weight of the additive used was 5100 g/mol (below 8000 g/mol).

After establishment of continuous conditions, a portion of the suspension of crystals is recovered and is filtered on a Buchner filter and filter paper. The filtered solution comprises about 125 grams of sodium bicarbonate per kilogram of aqueous solution and 65 grams of sodium carbonate per kilogram of aqueous solution. The sodium bicarbonate particles obtained on the Buchner are washed with 250 ml of ethanol and then dried in the same way as in example 1. The powder of particles thus obtained is characterized. The following measurements are performed:

-   -   Undersize (passing through) on a sieve at 125 μm: (wt %)     -   Oversize (retained) on a sieve at 250 μm: (wt %)     -   Laser granulometry (D10, D50, D90 and span) (μm)     -   Bulk density (BD) (kg/m³ or g/L)     -   Dissolution time (s)     -   Angle of repose)(°)     -   Flow test (tube number in the ‘Degussa’ test)     -   Abrasion index (AI in %).         Procedure for Measurement of the Dissolution Time:

The dissolution time (comparative between the different samples) is measured with a conductometric cell immersed in a stirred beaker with 1 liter of demineralized water maintained at 25° C. (±1° C.) Stirring is carried out with a marine propeller at 350 revolutions per minute. 10 grams of sodium bicarbonate particles are introduced into the solution, where they dissolve and cause an increase in conductivity of the solution. The dissolution time corresponds to the time taken to reach 95% of the final value of the conductivity in solution.

Procedure for Measurement of the Angle of Repose:

The angle of repose of the powders obtained is measured after formation of a poured cone that the powder forms on falling from a sieve of size 710 μm onto a cylinder with diameter of 50 mm and height of 80 mm. The height of the sieve relative to the top of the cone must be maintained between 2 and 3 cm. The angle of repose AR) (°) is calculated from measurement of the height H (mm) of the heap of powder remaining on the cone: [AR]=tan⁻¹(H/28)*(180/π) Procedure for Measurement of Flowability by the Flow Test:

Measurement of flowability consists of determining the flow of a sample of powder contained in a conical container, through an orifice of calibrated diameter. Seven cones, with height of 125 mm and diameter of 43.5 mm at the top have an orifice with increasing flow diameter: 2.4 mm (tube 1), 4.9 mm (tube 2), 8.0 (tube 3), 9.8 mm (tube 4), 12.3 (tube 5), 15.2 (tube 6) and 18.1 mm (tube 7). The test can determine the minimum opening (tube 1 to 7) starting from which the product can flow.

Procedure for Measurement of the Duration of the Abrasion Index:

The abrasion index at 63 μm consists of the difference of the relative measurement of weight of powder below 63 μm before and after abrasive wear. The wear test consists of passing 100 grams of powder through a rotating drum made of stainless steel with an inside diameter of 150 mm, internal length of 22 cm, and equipped with 3 blades with width of 35 mm. The drum rotates at 40 revolutions per minute for 20 minutes.

Remarks:

The following tables (Table 9.a and 9.b) present the results obtained: The tests with additives referenced (a) carried out at a stirring speed of 800 rev/min or referenced (b) carried out at 1070 rev/min must be compared with the respective reference tests without additive:

-   -   test 1(a) at 800 rev/min     -   test 2(b) at 1070 rev/min.         Moreover, measurement of the specific surface areas (BET surface         areas) on the sodium bicarbonate particles obtained in the tests         without additives: 1(a) and 1(b) and test 6(a) (sodium         polyacrylate at 500 ppm) gives the following values:     -   test 1(a) and test 2(b) (without additive): BET between 0.05 and         0.15 m²/g,     -   test 6(a) (sodium polyacrylate at 500 ppm): 1.4 m²/g.         It should be pointed out that notably in carbonation:     -   Polyacrylate (average molecular weight: 5100 g/mol) added at 5,         10, 50 and 500 ppm, and sodium hexametaphosphate at 50 ppm lower         the granulometry of the bicarbonate particles and the BD. The         angles of repose are increased. The dissolution times are         decreased unless there is a large addition of polyacrylate at         500 ppm—Test 6(a) where the dissolution time is increased.     -   Phosphates: sodium hexametaphosphate at 50 ppm lowers the         granulometry of the bicarbonate particles and the BD.     -   Sodium sulfate at 500 ppm increases the granulometry of the         particles and increases the BD.     -   Polysaccharides such as alginic acid at 10, 20 and 50 ppm and         carboxymethylcellulose at 20 ppm (tests 9 to 12) increase the         granulometry of the particles, reduce the span, generally         increase the BD, and increase the dissolution time. The abrasion         index is improved with a decrease in fines <63 μm generated         during the wear test. Photographs of the sodium bicarbonate         particles, taken with the light microscope, indicate a more         compact, parallelepipedal shape in the presence of         polysaccharides whereas the bicarbonate particles without         additive are of a more elongated, needle-like shape.     -   Betaine (quaternary amine) at 10, 20 and 50 ppm increases the         granulometry of the sodium bicarbonate particles, reduces the         span, increases the dissolution time, and increases the abrasion         index.

In the case when disclosure of the patents, of the patent applications and of publications that are incorporated here by reference would generate a conflict in the understanding of a term making it unclear, the present descriptive specification takes precedence.

TABLE 9.a Example 3 - Results of the carbonation tests (F <125 μm, F >250 μm, D10, D50, D90, and span of the sodium bicarbonate particles obtained) Stirring Fraction Fraction Additive speed <125 μm >250 μm D10 D50 D90 Test Additive (mg/kg) (rev/min) (% w/w) (% w/w) (μm) (μm) (μm) SPAN 1(a) No additive (Ref. a) — 800 7 63 138 290 504 1.26 2(b) No additive (Ref. b) — 1070 24 33 75 197 329 1.29 3(a) Sodium polyacrylate 5 800 12 62 114 301 357 1.50 4(a) Sodium polyacrylate 10 800 23 47 70 237 433 1.53 5(a) Sodium polyacrylate 50 800 18 54 5 35 129 3.52 6(a) Sodium polyacrylate 500 800 18 64 3 43 563 12.93 7(a) Sodium hexametaphosphate 50 800 17 63 9 53 133 2.36 8(b) Sodium sulfate 500 1070 13 33 113 207 320 1.00 9(b) Alginic acid 10 1070 13 41 111 224 339 1.02 10(a)  Alginic acid 20 800 8 62 131 283 464 1.18 11(b)  Alginic acid 50 1070 10 33 126 215 328 0.94 12(a)  Carboxymethylcellulose 20 800 5 76 159 350 564 1.16 13(b)  Soybean lecithin 20 1070 22 7 90 180 246 0.86 14(b)  Betaine 10 1070 8 67 137 300 477 1.13 15(b)  Betaine 20 1070 10 39 124 227 309 0.82 16(b)  Betaine 50 1070 7 60 137 274 416 1.02

TABLE 9.b Example 3 - Results of the carbonation tests (continued) (BD, Dissolution time, Angle of repose, Flowability of flow, Abrasion index) Stirring Angle Flowability Abrasion Additive speed BD Dissolution of repose Flow index at Test Additive (mg/kg) (rev/min) (kg/m³) time (s) (°) (tube No.) 63 μm (%) 1(a) No additive (Ref. a) — 800 908 33 34 2 0.13 2(b) No additive (Ref. b) — 1070 732 22 41 2 0.95 3(a) Sodium polyacrylate 5 800 796 35 40 2 1.45 4(a) Sodium polyacrylate 10 800 511 19 41 5 1.68 5(a) Sodium polyacrylate 50 800 324 17 63 8 0.97 6(a) Sodium polyacrylate 500 800 415 43 57 — — 7(a) Sodiumm hexametaphosphate 50 800 337 14 65 — 2.05 8(b) Sulfate sodium 500 1070 851 26 37 1 0.76 9(b) Alginic acid 10 1070 961 31 37 1 0.91 10(a)  Alginic acid 20 800 724 34 39 2 — 11(b)  Alginic acid 50 1070 990 30 36 1 0.51 12(a)  Carboxymethylcellulose 20 800 585 36 41 3 — 13(b)  Soybean lecithin 20 1070 871 26 39 1 0.75 14(b)  Betaine 10 1070 717 28 39 2 1.47 15(b)  Betaine 20 1070 773 25 39 1 0.96 16(b)  Betaine 50 1070 736 26 39 2 1.03 

The invention claimed is:
 1. A method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate in the presence of an additive in the solution, wherein the additive is a polycarboxylate selected from the group consisting of: polyacrylates, crosslinked polyacrylates, copolymers of acrylic and maleic acids, copolymers of sodium prop-2-enoate, and alkali metal salts thereof; wherein the additive is present in the solution at a concentration of at least 1 ppm; and wherein the additive has an average molecular weight below 8000 g/mol.
 2. The method as claimed in claim 1, wherein the additive is present at a concentration of at most 200 ppm.
 3. The method as claimed in claim 1, wherein crystallization of alkali metal bicarbonate particles is carried out by cooling the solution.
 4. The method as claimed in claim 3, wherein cooling of the solution is carried out between 70° and 30° C.
 5. The method as claimed in claim 1, wherein crystallization of alkali metal bicarbonate particles is carried out by carbonation of the solution with carbon dioxide.
 6. The method as claimed in claim 5, wherein carbonation of the solution is carried out at a temperature of at least 20° C.
 7. The method as claimed in claim 1, wherein crystallization of alkali metal bicarbonate particles is carried out by evaporation of at least a proportion of the solution.
 8. The method as claimed in claim 1, wherein crystallization of the alkali metal bicarbonate particles is carried out by concomitant cooling and carbonation of the solution; or by concomitant cooling and carbonation and evaporation of the solution.
 9. The method as claimed in claim 5, wherein carbonation of the solution with carbon dioxide is carried out with a gas comprising: 10 to 100% of CO₂ by volume based on dry gas.
 10. The method as claimed in claim 1, being carried out at a temperature between 20° C. and 95° C.
 11. The method as claimed in claim 1, wherein addition of the additive to the solution is carried out for controlling or modifying: the granulometry or the span, or the undersize or oversize on a sieve at 125 μm, or the undersize or oversize on a sieve at 250 μm, of the alkali metal bicarbonate particles obtained after separation of the crystallization solution and of the alkali bicarbonate particles.
 12. The method as claimed in claim 1, wherein addition of the additive to the solution is carried out for controlling or modifying: the dissolution time, or the bulk density (BD), or the flow rate in a calibrated orifice, or the abrasion index, of the alkali metal bicarbonate particles obtained after separation of the crystallization solution and of the alkali bicarbonate particles, then drying of the alkali metal bicarbonate particles.
 13. The method as claimed in claim 3, wherein addition of the additive to the solution makes it possible to: reduce the fraction of sieve oversize at 250 μm, or reduce the bulk density (BD) of the alkali metal bicarbonate particles relative to one and the same preparation effected without the additive.
 14. The method as claimed in claim 5, wherein the additive is present at a concentration of at most 200 ppm, and wherein addition of the additive to the solution makes it possible to: reduce the granulometry or increase the span, or increase the sieve undersize at 125 μm, or reduce the dissolution time, or reduce the bulk density (BD), or increase the abrasion index, of the alkali metal bicarbonate particles relative to one and the same preparation effected without the additive.
 15. The method as claimed in claim 5, wherein the additive is present at a concentration of about 500 ppm, and wherein addition of the additive to the solution makes it possible to: reduce the granulometry, or increase the span, or increase the sieve undersize at 125 μm, or increase the dissolution time, or reduce the bulk density (BD), of the alkali metal bicarbonate particles relative to one and the same preparation effected without the additive.
 16. Alkali metal bicarbonate particles obtainable by the method as claimed in claim 1 and comprising at least 10 ppm of the additive.
 17. The method as claimed in claim 1, wherein the additive is present at a concentration of at most 100 ppm.
 18. The method as claimed in claim 1, wherein the additive is present at a concentration of at most 70 ppm. 